Disk drives are used to store electronic data on a magnetically recordable disk medium, providing both read and write access via one or more heads that are disposed near the rotating disk. In certain disk drives, a coil is used to magnetize selected portions of the disk based on data being written to the drive, and a magnetoresistive sensor head is used to read data from the disk. Pre-amplifier circuitry is provided in the disk drive system to interface a host system with the read and write heads where the pre-amplifier is typically an integrated circuit with a connector for interconnection with a channel circuit or “channel chip” of the host system via a flexible cable. During data writing operations, data from the host system is provided to writer circuitry on the pre-amplifier for actuating the write coil. For reading operations, sensor signals from the read head are provided to one or more amplification or gain stages of the pre-amplifier circuit, and a pre-amp output stage includes driver circuitry to provide suitable data signals through the flexible cable to the host system. Problems may occur, however, if the common mode voltage at the driver outputs varies, particularly when switching between reading (or amplifying) and writing modes. For example, ever-increasing data rates reduce the time for the channel chip to recover when the pre-amplifier driver output circuit transitions from low impedance during read mode to high impedance for write mode operation, and recovery is hampered by significant changes in the pre-amplifier common-mode output voltage. In addition, system offset information is useful for dynamic range and distortion testing, and it may be desirable to measure other signals during testing. However, the pre-amplifier output stage typically includes input capacitors to remove DC offsets and to reference the driver output voltage to appropriate circuit ground for interfacing with the channel chip, and the pre-amplifier offset blocking capacitors prevent offset or other test signal measurement at the driver outputs. Accordingly, there is a need for improved disk drive pre-amplifier circuitry to facilitate offset measurements during testing and to mitigate output common mode voltage variations during operation.